All the baths previously used industrially for this type of deposition, and regardless of whether they are acid baths or alkaline baths, make use of a reducing agent containing phosphorous or boron in the form of an alkaline hylpophosphite or hydrogenated derivatives of boron. In addition to the reducing agent, they contain at least one compound of the metal(s) to be deposited, at least one complexing agent for said metal, and at least one stabilizing agent.
The deposits of nickel and/or cobalt obtained from such baths are not pure. They contain phosphorous or boron from the reducing agent together with elements from the stabilizing agents: sulfur and/or heavy metals, e.g. thallium.
ONERA's published French patent application No. 2 531 103 describes baths for obtaining deposits which are free from impurities coming from the stabilizing agents, and which are intended, in particular, for treating turbine blades for aeronautical applications where such impurities are highly undesirable and cannot be removed by subsequent treatments.
However, the deposits obtained still contain phosphorous or boron from the phosphorous or boron reducing agents and these impurities are also unwanted in the applications concerned; they must therefore be removed after deposition.
Unfortunately, it is very difficult (and even impossible in some cases) to eliminate phosphorus. And although boron can be removed, for example by the method described in ONERA's published French patent application No. 2 278 794, such removal is not complete.
There thus exists a need for chemical deposition baths in which the reducing agent contains neither phosphorous nor boron so as to avoid said impurities being present in the deposit.
Hydrazine satisfies these conditions: when oxidized by ions of nickel or cobalt it gives rise solely to hydrogen and nitrogen, both of which escape in the gaseous state.
Baths for chemically depositing nickel and/or cobalt and including hydrazine as a reducing agent are described in an article by Dini and Coro published in "Plating" vol. 54, p. 385 (1967), and in U.S. Pat. No. 3,198,659.
The baths described in the article require the use of very high purity starting products, and maintaining such baths is extremely difficult, so they are too expensive to be used to develop the method on an industrial scale.
The baths described in U.S. Pat. No. 3,198,659 are capable solely of providing very thin deposites as mentioned in the article which refers to said patent, with the deposit thickness being about one micron.
An aim of the invention is to provide baths capable of chemically depositing nickel and/or cobalt on an industrial scale and also to enable very pure deposits of oonsiderable thickness to be obtained.
Another aim of the invention is to provide hydrazine baths capable of chemically depositing nickel and/or cobalt on an industrial scale and of giving rise to deposits of considerable thickness.
In order to solve this problem, the inventors have studied the equilibria and the chemical reactions which obtain in hydrazine baths, and as a result of this work they have formulated the following hypotheses.
The nickel and/or cobalt ions are complexed concurrently both by the hydrazine and by the complexing agent per se in the bath, with the fractions complexed by the hydrazine and by the complexing agent being determined by the dissociation constants of the two complexing reactions.
The nickel and/or cobalt is deposited solely by discharge from the hydrazine/metal cation compIex.
Numerous ions, and in particular chloride, sulfate and nitrate ions, have a disturbing effect on the formation of the hydrazine/metal cation complex and/or on the catalytic activity of the deposit.
These considerations have pointed the inventors on the path to a solution.
The present invention provides a bath for chemically depositing nickel and/or cobalt, the bath comprising a compound of the metal(s) to be deposited, a reducing agent, at least one complexing agent for the metal, and at least one stabilizing agent.